Potential Nitrate Leaching Under Common Landscaping Plants

Studies on N losses from ornamental plantings - other than turf - are scant despite the ubiquity of these landscaping elements. We compared pore water NO₃ and extractable soil NO₃ and NH₄ in areas with turf, areas with seven different types of ornamental landscape plantings, and a native woodland. Turf areas received annual N inputs of ~48 kg ha-¹ and annual flowers received ~24 kg N ha-¹ at the time of planting. None of the other areas were fertilized during the course of the study. Data were collected on 23 occasions between June 2002 and November 2003. Pore water NO₃ concentrations at a 60-cm depth - based on pooled data - were highest (1.4 to 7.8 mg NO₃-N l-¹) under ground covers, unplanted-mulched areas, turf, deciduous trees, and evergreen trees, with no differences among these vegetation types. Lower values were observed under woodlands, annual and perennial flowers, and evergreen and deciduous shrubs. Pore water NO₃ concentrations exceeded the drinking water regulatory limit of 10 mg NO₃-N l-¹ under ground covers, turf and unplanted-mulched areas in 39, 20 and 10% of samples, respectively. Leaching losses of NO₃-N over 18 months ranged from 0.17 kg N ha-¹ in the woodlands to 34.97 kg N ha-¹ under ground covers. Annual NO₃ losses under unplanted-mulched areas and ground covers were approximately twice the average N input (10 kg N ha-¹ year-¹) from atmospheric deposition. Extractable NO₃ in woodland soils (0.5 μg NO₃-N g-¹) was lower than for all other vegetation types (3.1-7.8 μg NO₃-N g-¹). Extractable NH₄ levels were highest in woodlands, deciduous trees, and annual flowers (6.7-10.1 μg NH₄-N g-¹). Most vegetation types appear to act as net N sinks relative to atmospheric inputs, whereas unplanted-mulched areas and areas planted with ground covers act as net sources of NO₃ to groundwater.